It has been recognized for many years that the hybridization of closely related plants results in offspring having a combination of desirable traits which previously were possessed separately by the parent plants. Certain hybrid plants also have possessed a vigor or heterosis which has rendered them of considerable economic importance.
While significant advances have been made in the production of hybrid sugar beets, hybrid corn (see U.S. Pat. No. 2,753,663 to Jones), hybrid sorghum, and hybrid alfalfa (see U.S. Pat. No. 3,570,181 to Davis) many economically important crops remain in which no commercially practicable breeding technique has been developed for the production of a hybrid. Obviously cross-pollination carried out by hand is not feasible for commercial production. Much of the difficulty experienced when attempts have been made to develop a hybrid of many crops can be traced to the diverse reproduction systems and modes of pollination encountered. Accordingly, each crop must be approached separately and its unique characteristics taken into consideration.
Soybean plants (i.e. Glycine max plants) are recognized to be naturally self-pollinated plants which while being capable of undergoing cross-pollination rarely do so. Insects are reported by some researchers to carry pollen from one soybean plant to another but it generally is estimated that less than one percent of soybean seed formed in an open planting can be traced to cross-pollination, i.e., less than 1 percent of the soybean seed formed in an open planting is capable of producing F.sub.1 hybrid soybean plants. See the articles by Elbert R. Jaycox entitled "Ecological Relationships between Honey Bees and Soybeans" appearing in the American Bee Journal, Vol. 110(8):306-307 (August 1970), Vol. 110(9):343-345 (September 1970) and Vol. 110(10):383-385 (October 1970).
The relatively low proportion of cross-pollination commonly observed in soybean plants when grown in nature can be traced to the characteristic floral configuration exhibited by soybean plants. The pistillate (female) and staminate (male) elements of soybean flowers are normally present on the same plant and are located within perfect flowers. The opening of the individual soybean flowers (florets) is believed to be triggered by the length of time the plant is exposed to light. However, the anthers and stigma continue to be tightly enclosed within petals (i.e. the portion of the flower known as the keel petals). When dehiscence of another tissue occurs and pollen is shed from the anthers, it tends immediately to contact the stigma in the same floret and is retained there by the keel petals. A seed pod ultimately is formed from this fertilization assuming that the pollen does not abort.
Some researchers have reported the existence of a degree of male sterility in selected soybean plants. See, for instance:
1. "A Partially Male Sterile Strain of Soybeans", by C. E. Caviness, H. J. Walters, and D. L. Johnson, Crop Science, Vol. 10, p 107-108, (Jan. - Feb. 1970), PA1 2. "Inheritance of a Male-Sterile Character in Soybeans", C. A. Brim and M. F. Young, Crop Science, Vol. 11, p 564-566, (July - Aug. 1971), and PA1 3. U.S. Pat. No. 3,903,645.
The last named reference disclosed the production of F.sub.1 hybrid soybeans by growing a population of small seeded soybean plants of the open floret (exposed floral stigma) type in pollinating proximity to large seeded soybean plants having normal (closed) flowers. The reference further discloses that the open floret plants are preferably partially male sterile, i.e. characterized by delayed pollen release. It also discloses that process is conducted by random cross-pollination by pollen carrying insects, including honey bees, bumble bees, solitary bees, thrips and leaf-cutter bees.
Now it has been discovered that such F.sub.1 hybrid soybeans can be produced on a commercial scale without recourse to the relatively rare open florate plants if leaf-cutter bees are provided for the cross-pollination and if the variety selected as the female parent exhibits delay pollen release.